Silicon is abundant in nature and is receiving attention as an eco-friendly and high-capacity anode material having theoretical capacity of as much as 4200 mAh/g. However, commercialization of silicon-based materials has been hindered by low electric conductivity and large volume change of 400% during cell operation, which leads to a drastic capacity decrease. Therefore, there has been recently suggested a method of improving low electric conductivity of silicon electrodes by producing a silicon-carbon composite using a pyrolysis method or the like, or by using a conductive polymer binder (see FIG. 1). While such method may improve high rate capability by improving electric conductivity, the method is difficult to perform and has a limited effect of improving cycle life since a conductive material is separated due to a volume change.
There is another method of improving the performance of a silicon electrode by synthesizing silicon in the form of nanotubes to suppress the effect caused by volume change (see FIG. 2). However, the method is also difficult to perform and to commercialize.
Further, there is an attempt to reduce the effect of volume change by reducing the size of silicon particles or by using a host material, such as graphite. The method is particularly useful relative to other methods that change the structure of silicon, but also has problems of high production costs and deteriorated cycle life characteristics during a long-term charging/discharging process.
The above methods focus on modification of materials directly related to cycle life deterioration. However, since the methods may not be a fundamental solution to suppress volume expansion of silicon, a binder, which is another material in an electrode, is currently receiving attention. A binder binds various materials in an electrode and is affected the most by physical stress caused by the volume change of a silicon-based material. Recent studies have found that by strengthening physical properties of a binder material, separation of a conductive material may be prevented, and pulverization and isolation of silicon particles may be suppressed, thereby improving life cycle characteristics of a battery. From the viewpoint of improving the physical properties of a polymer binder, researchers have conducted much research on various materials, such as poly acrylic acid (PAA), carboxymethyl cellulose (CMC), alginate, conductive polymers, and the like, to be used as a binder material instead of poly(vinylidene fluoride) (PVDF) that is commonly used as a binder material for battery. They have come to a conclusion that, a polymer having a polar functional group, such as —OH or —COOH, which can strongly interact with Si—OH (silanol) that is an oxide film on the surface of silicon particles, is suitable as a next generation binder material.
By introducing a polar functional group for a binder material, adhesive strength with a silicon material may be enhanced, and separation of silicon particles may be suppressed. However, even by using a polar polymer, deterioration of a binder material caused by volume change during cell operation may not be prevented, and a problem with long-term cell performance still remains. Accordingly, in order to prevent deterioration of a polymer material caused by the volume change, a cross-linking system has been recently introduced in which by cross-linking a polymer material, physical properties and cell performance have been improved. However, the system also has problems in that long-term cell performance may not be ensured, since physical stress due to a continuous volume change leads to deterioration of the crosslinking. As an irreversible cross-linking system, Korean Patent Publication No. 10-2015-0000063 discloses “an optically cross-linked poly(acrylic acid) binder for silicon anode”.
Accordingly, there is a need for a binder material for a cross-linking system in which physical properties may be recovered and maintained by having strong binding force and rebinding may be enabled even when binding is broken.
Patent Documents cited in Background section include: (1) Korean Patent Publication No. 10-2015-0000063 and (2) Korean Patent Publication No. 10-2014-0117313.